The goal of the proposed research is to provide a better understanding of the normal anatomical and behavioral development of the motor system, and to determine how this developing neural system responds to early lesions of its major efferent pathway, the pyramidal tract. The research outlined in this proposal deals specifically with the following aspects of this goal, (a) the pre-and post-natal proliferation, migration, and differentiation of neurons in the motor cortex as revealed by light microscopic tritiated thymidine autoradiography, (b) the normal post-natal development of the motor cortex and its afferent fibers as studied by the Golgi and Nissl methods, (c) the timing of the arrival of afferent fiber systems to the motor cortex as studied with anterograde and retrograde methods, (d) the developmental outgrowth of efferent fibers from the motor cortex (i.e. the pyramidal tract) into the spinal cord demonstrated by anterograde and retrograde techniques, (e) the age-related effects of early lesions of the pyramidal tract in the normal development and maintenance of the motor cortex, (f) the normal development of digital motor behavior and the effects of early pyramidal tract lesions on fine manipulatory movements of the forepaw. The hamster will be used as the experimental animal in all aspects of this study.